This device relates to a nuclear fuel assembly having a stress relieving fastener and more particularly to a nuclear fuel assembly having a stress relieving fastener for relieving thermally induced stresses developed in a nuclear fuel assembly when the fuel assembly is disposed in a nuclear reactor and when the nuclear reactor is brought to its operating temperature.
It is well known that when a solid member is subjected to a change in temperature it will either expand or contract. If the expansion or contraction is prevented or restrained, thermal stresses will develop in the member. The thermal stresses developed in the member will be a function of its geometry and the temperature distribution within the member. When the member is disposed in a nuclear reactor, the member may undergo changes in temperature distribution and thus may experience thermal stresses either during reactor startup, shutdown, transient conditions or non-steady state reactor operation.
A nuclear reactor is a device for generating heat from the controlled fission of nuclear material such as uranium or plutonium bearing compounds. The nuclear material is typically in the form of fuel pellets that are housed in cylindrical fuel rods which are traditionally arranged in a square pitch array. The fuel rods are in turn disposed in the fuel assembly which is a means for containing the fuel rods. A plurality of fuel assemblies are disposed in the nuclear reactor. The fuel assembly generally comprises a top nozzle, a channel and a bottom nozzle. The top nozzle is mounted on the channel by studs which are attached to the channel. The studs and channel may be formed from a material such as Zircaloy and the top nozzle may be formed from a different material such as stainless steel. Each stud may be attached to the top nozzle by a fastener which may be 316 stainless steel.
In the operating temperature range of the typical nuclear reactor, the coefficient of linear thermal expansion, commonly referred to in the art as the thermal expansion rate, for stainless steel is approximately three times that of Zircaloy. Therefore, the thermal expansion rate for the stainless steel top nozzle is approximately three times that of the Zircaloy studs. Hence, when the nuclear reactor is brought to its operating temperature, differential thermal expansion of the studs and top nozzle may produce potentially undesirable high thermal stresses in the studs. Consequently, a means is needed to mitigate these potentially undesirable thermal stresses.
Traditional solutions such as the use of a belleville washer are not feasible due to the limited space available in the reactor and due to the undesirable potential of additional, small loose parts becoming separated from the fuel assembly during fuel assembly reconstitution.
One prior art solution for relieving potentially undesirable thermal stresses was employed in demonstration assemblies delivered to an operating nuclear power plant in 1985. This solution entailed placing a carefully controlled gap between a fastening nut and the top nozzle. In this manner the top nozzle was allowed to expand without restraint during reactor heatup due to the presence of the gap; hence, the gap mitigated development of potentially undesirable thermal stresses. However, achieving the proper size of the gap was difficult to control particularly if the gap sizing operation was performed remotely and under water. Although this solution can be most conveniently used for fresh, non-irradiated fuel assemblies, a more convenient solution is needed for irradiated assemblies. A more convenient solution is particularly needed in the instance of irradiated assemblies where it is necessary to remotely attach the fastening nut to the stud under water.
There are several devices known in the art for attaching two or more members by utilizing a fastening nut; however, many of these devices are not appropriate for remotely attaching a nuclear fuel assembly top nozzle to a fuel assembly channel such that thermal stresses are relieved during reactor heatup.
One such device known in the art for attaching two or more members by utilizing a fastening nut is disclosed by U.S. Pat. No. 4,189,348 issued Feb. 19, 1980 in the name of Harry A. Donck et al. and entitled "Fuel Rod Assembly To Manifold Attachment" which is assigned to the General Atomic Company. This patent discloses a fuel rod assembly having fuel rods and an upper sealing surface. The fuel rods extend through openings in the sealing surface and are held in place by cap nuts. Each nut has a deformable portion for locking each rod to the sealing surface to prevent loosening of the nut. In an alternative embodiment of this device, the nut uses a deformable flange which projects radially outward from the base of the nut for locking the nut and a beam. The function of this device is to seal gaseous fission products and to allow the flow of these fission products from the fuel rods to a vent manifold passageway.
Another device known in the art for attaching two or more members by using a fastening nut is disclosed by U.S. Pat. No. 4,499,047 issued Feb. 12, 1985 in the name of Bo Borrman et al. and entitled "Fuel Assembly With A Top Plate Arranged Above The Upper End Surfaces Of The Fuel Rods" which is assigned to AB Asea-Atom. This patent discloses a fuel assembly with a top plate arranged above the upper end surfaces of the fuel rods. The top plate is secured in place by a helical spring and a nut. A locking washer is arranged between the helical spring and the nut to increase resiliency in the movement of the upper plate. Therefore, this device utilizes a nut and washer that do not have deformable portions specifically capable of relieving thermally induced stresses.
A fastener having a flexible portion is disclosed by U.S. Pat. No. 4,269,248 issued May 26, 1981 in the name of Barry L. MacLean et al. and entitled "Fastener With Flexible Flange" which is assigned to the MacLean-Fogg Company. This patent discloses a fastener having a nut portion with a bearing surface defined at least in part by a flange that is bowed and that extends radially outward from the base of the nut. Segments of the bearing surface and flange are flexible and resilient so that when the fastener is tightened against a workpiece, deflection of the resilient segments compensates for loss of tension. Therefore, this device comprises a fastener wherein each resilient flange segment is bowed and extends radially outward from the base of the nut for securely tightening the fastener against a workpiece.
U.S. Pat. No. 2,375,325 issued May 8, 1945 in the name of Edward Roker Robertson assignor to Oliver Edwin Simmonds and entitled "Self-Locking Nut" discloses a nut having a flexible diaphram portion. This two-piece nut comprises a nut portion and a locking member which is formed separately from the nut portion wherein the locking member has a flexible diaphram. Therefore, this device has more than one element, wherein one of the elements is the flexible diaphram for securing the nut and a bolt tightly together. Consequently, if disposed in a nuclear reactor, the two-piece construction of this device has the undesirable potential for producing a loose part in the reactor should the two pieces comprising the device separate.
A sealing nut having a compressible insert is disclosed by U.S. Pat. No. 4,126,170 issued Nov. 21, 1978 in the name of Lon DeHaitre and entitled "Sealing Nut With Preformed Turtleneck Insert" which is assigned to the Abbott Screw and Manufacturing Company. This patent discloses a sealing nut capable of withstanding high pressures and having a compressible insert disposed in its working face which contacts a bearing surface when the nut is engaged on a stud. The nut is for locking and sealing the stud in place. The compressible portion of this device is not integrally formed with the nut body and the function of the nut is primarily to seal and lock rather than primarily to relieve thermally induced stresses. Consequently, if disposed in a nuclear reactor, this device has the undesirable potential for producing a loose part in the reactor should the insert become disengaged from the nut body.
U.S. Pat. No. 1,156,798 issued Oct. 12, 1915 in the name of John W. Meaker entitled "Nut" discloses a nut having a flange. In this patent the nut has a flange on its working face which contacts a bearing surface. The flange portion of this device is turned outwardly from the nut body.